Existing design guidelines for concrete hinges consider bending-induced tensile cracking, but the structural behavior is oversimplified to be time-independent. This is the motivation to study creep and bending-induced tensile cracking of initially monolithic concrete hinges systematically. Material tests on plain concrete specimens and structural tests on marginally reinforced concrete hinges are performed. The experiments characterize material and structural creep under centric compression as well as bending-induced tensile cracking and the interaction between creep and cracking of concrete hinges. As for the latter two aims, three nominally identical concrete hinges are subjected to short-term and to longer-term eccentric compression tests. Obtained material and structural creep functions referring to centric compression are found to be very similar. The structural creep activity under eccentric compression is significantly larger because of the interaction between creep and cracking, i.e. bending-induced cracks progressively open and propagate under sustained eccentric loading. As for concrete hinges in frame-like integral bridge construction, it is concluded (i) that realistic simulation of variable loads requires consideration of the here-studied time-dependent behavior and (ii) that permanent compressive normal forces shall be limited by 45% of the ultimate load carrying capacity, in order to avoid damage of concrete hinges under sustained loading.

Teil 1: Kleinkörperversuche Stichworte Hochfester Aufbeton; Verbundfuge; Abscherversuch; Haftzugversuch; Kerb-Spaltzugversuch; Rauigkeit; adhäsive VerbundschubfestigkeitHigh strength concrete overlays -Part 1: small scale tests Small scale and member tests with different specimen geometries and test setups were performed to investigate the adhesive bond along interfaces between normal strength concrete and high strength overlay. While small scale tests were in the focus of the first series of the research project, in the second series member tests were performed as well as series of small scale tests with an optimized recipe for the high strength concrete layer (providing better workability and higher compressive strength). In comparison small scale tests with normal strength overlay were performed. In general the small scale tests provide consistent results with rather low scatter for same test-setups and roughness category. The improved mix proportion for the high strength concrete had a significant effect on the bond strength results. In general the bond strength values were significantly higher than observed with normal strength overlay.

Six different concretes are characterized during material ages between 1 and 28 days. Standard tests regarding strength and stiffness are performed 1, 3, 7, 14, and 28 days after production. Innovative three-minute-long creep tests are repeated hourly during material ages between one and seven days. The results from the standard tests are used to assess and to improve formulas of the fib Model Code 2010: the correlation formula between the 28-day values of the strength and the stiffness, and the evolution formulas describing the early-age evolution of the strength and the stiffness during the first four weeks after production. The results from the innovative tests are used to develop a correlation formula between the 28-day values of Young’s modulus and the creep modulus, and an evolution formula describing the early-age evolution of the creep modulus during the first four weeks after production. Particularly, the analyzed CEM I concretes develop stiffness and strength significantly faster than described by the formulas of the Model Code. The creep modulus of the investigated concretes evolves significantly slower than their strength and stiffness. Thus, concrete loaded at early ages is surprisingly creep active, even if the material appears to be quite mature in terms of its strength and stiffness.

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